Paper industry continuously strives to improve paper and paperboard quality, increase process speeds, reduce manufacturing costs etc. Various chemicals, synthetic and naturally occurring, are used to treat pulp in order to improve, for example, retention and drainage, and to create physical properties such as wet and dry strength of the final paper product.
A retention agent is a process chemical that improves retention of a functional chemical in a substrate. The result is that totally fewer chemicals are used to get the same effect of the functional chemical and fewer chemicals goes to waste.
Drainage additives are materials that increase drainage rate of water from pulp slurry on a wire. Common drainage additives are cationic starch and polyacrylamide.
Wet strength additives ensure that when paper becomes wet, it retains its strength. This is especially important in a tissue paper. Examples of wet strength additives are urea-formaldehyde (UF), melamine-formaldehyde (MF) and polyamide-epichlorohydrin (PEA).
Dry strength additives are chemicals that improve paper strength of normal or not wet condition. Typical chemicals used are starch and polyacrylamide (PAM) derivatives. The starch and PAM derivatives may be anionically or cationically charged. By using cationic starch or PAM, negatively charged fibers can bind with the cationic starch or PAM and thus increase inter-connections between the fibers, and thus strength.
For example, U.S. Pat. No. 8,647,470 discloses a method for producing paper, paperboard and cardboard having improved dry strength. The improved dry strength is obtained by adding into a stock an aqueous blend comprising nanocellulose and at least one polymer selected from the group consisting of the anionic polymers and water-soluble cationic polymers, draining of the paper stock and drying of the paper products.
An Interpenetrating Polymer Network (IPN) is a polymer, also referred to as IPN material, comprising two or more networks which are at least partially interlaced on a molecular scale, but not covalently bonded to each other. The network cannot be separated unless chemical bonds are broken. The two or more networks can be envisioned to be entangled in such a way that they are concatenated and cannot be pulled apart, but not bonded to each other by any chemical bond.
In other words, the interpenetrating polymer networks are a combination of at least two polymers, wherein at least one of the polymers is synthesized (polymerized) and/or cross-linked in the immediate presence of the other(s).
Simply mixing two or more polymers does not create an interpenetrating polymer network (but a polymer blend), nor does creating a polymer network out of at least one kind of monomer(s) which are bonded to each other to form one network (heteropolymer or copolymer).
Document Chang et al., Polymers for Advanced Technologies (2011), 22(9), 1329-1334, discloses structure and properties of cellulose/poly(N-iso-propylacrylamide) double network hydrogels prepared by IPN method. The cellulose hydrogel are prepared by chemically crosslinking cellulose in NaOH/urea aqueous solution, which is employed as first network. Second network is subsequently obtained by in situ polymerizing/crosslinking of N-iso-propylacrylamide in the cellulose hydrogel. The method creates double network hydrogel, which combines natural polymer and synthesized poly(N-isopropylacrylamide collectively in one system.
Even though there is available cellulose containing IPNs, there is still a need for novel cellulose containing IPN materials to be used as additives in production of paper and paperboard having improved properties.